I have very little experience in print, if someone could give me some advice on this matter: I have to illustrate a one sheet movie poster (27"*40"), and I was wondering if I can get good results (meaning good quality for print) working on a less bigger piece of paper, and resizing it to poster size later in photoshop. I don't think there'd a problem if I scan it in 300 dpi? It's just that I find it terribly difficult to get proportions right working on large pieces.
Work at whatever feels comfortable to you if you are working with pen and paper. Then scan it as large as you can with as much PPI as you can.
When you scan the drawing, you can scan it at 500% or more and 1200PPI or more... which will allow you to properly size it for larger reproduction than the original drawing.
Of course, enlarging it when scanning may mean you need to clean it up a bit digitally to possibly sharpen it a bit.